The present invention relates to a system for purification and recirculation of water passing through the flow chamber of a flow cytometer.
A flow cytometer is an instrument for measuring the fluorescence and light scattering of individual biological cells in large numbers. The measurement is carried out when the cells are passed one by one through the focus of an intense source of light which excites fluorescent dyes in the cells. Thus, the fluorescence intensity is a measure of the particular constituent of the cell which binds the fluorescent dye. The intensity of the scattered light is primarily a measure of the size of the cell. The fluorescence and scattered light are collected by appropriate optics and detected by means of separate photomultiplier tubes or other sensitive light detectors. The detectors transform the light pulses into equivalent electrical pulses which are subsequently digitized, stored and classified in a computer.
The cells are carried through the excitation focus by a laminar flow of water having a cross-section of microscopic dimensions. This carrying water, which is commonly termed "sheath fluid", is fed into a conical flow chamber under a pressure of the order of 1 kg/cm.sup.2. The orifice of the flow chamber, which typically has a cross-section of the order of 100 .mu.m, leads either into air where the water forms a cylindrical jet or into a tube passing through the excitation focus. The sample, which is usually a suspension of cells, is introduced into the flow chamber through a tube along the center of the flow chamber. Hence, the cells are confined to the central core of the flow through the excitation focus.
The amount of sheath fluid passing through the flow chamber is about 10 ml/min, which means that a flow cytometer consumes on the order of 5 liters per day. Hence, most flow cytometers have a water reservoir of about that volume. This reservoir adds significantly to the weight and volume of the instrument. Especially in instruments intended for field use, it is important to reduce this volume as much as possible. In instruments designed for use in spaceships, such a reduction is of course essential. There is an increasing demand for field instruments which can run unattended for long periods of time.
The water used as sheath fluid must be of high purity. Usually one uses distilled water which is subsequently passed through a filter having a pore size of 0.2 .mu.m. Water of such quality is not readily available in all laboratories using flow cytometry.
Another problem in the use of flow cytometers is that some of the fluorescent dyes used to stain cells, and especially dyes used to stain the DNA, i.e. the genetic material, are highly mutagenic and/or carcinogenic. The waste water from flow cytometers should therefore not be drained into the public sewer, as is now the case. On the other hand it is expensive to dispose of such large volumes of water in a more responsible way.